There is a certain degree of satisfaction to be had once you hit a decent level from going back and destroying a few Entites. I really recommend trying when your party is up to it. As long as you intiate combat (and this is important, don't let it get the drop on you) you should win.

Once I got the Zodiac Spear (I was around level 30 or something), I immediately went after the one Entite that had nearly wiped out my party (and who seems to do this to a lot of people) - the Salamand Entite. It went down quite quick to a Berserk'd Spear-wielding Basch.